In the fall of 2006 Sharon Howell of San Francisco spotted a sign for Trivia Night at a local pub. The idea sounded appealing, so she e-mailed her co-workers at a public-relations firm, inviting them to go with her. As a relatively new employee, she did not know any of them well.

"Three other people from the office thought it would be fun, so we all went together," says Ms. Howell, a senior account executive for Lewis Global Public Relations. Today, 18 months later, the original foursome still attends every week, and others have joined them.

Americans are less likely than those in other parts of the world to socialize with their fellow employees beyond the workplace, according to researchers at the University of Michigan. They found that only 30 percent of employees have a close confidant at work, down from nearly half in 1985.

"Compared to counterparts in other countries, US co-workers are less likely to extend professional ties into a variety of settings beyond the workplace, even though factors that typically constrain social interaction with co-workers after hours – marriage, kids, time spent working – have remained constant or weakened," says researcher Aleksandra Kacperczyk.

Yet when Americans do socialize after work, they derive positive emotional energy from the experience, the study shows. Similarly, a survey by Accountemps finds that 57 percent of executives and nearly two-thirds of workers think office productivity improves when colleagues are friends outside the office.

Some after-hours gatherings have a specific purpose, such as celebrating a birthday, a promotion, or an engagement. Others simply offer an excuse to kick back, relax, and savor the end of a busy workday or workweek.

Family obligations prevent many people from taking part in after-hours gatherings. Other workers simply prefer to keep business and pleasure separate.

"Other than the odd lunch, I avoid these sorts of get-togethers," says Brian Olson, a vice president at Video Professor, Inc., in Lakewood, Colo. "I work with a terrific group of people each day, but I have another life outside work with my family. It's a good balance, and a safe one."

Still, he acknowledges that many get-togethers can be a good way to meet friends and bring different levels of the company together.

Socializing – or not – depends on the culture of a particular office, says Thom Singer, author of two books on the power of business relationships. "If people in your office do [socialize] and you choose not to, you can send a negative message that you don't want to play with others." Avoiding co-workers outside the office, he cautions, can be a "career-limiting" policy, while mingling socially can help pave the way to promotions and other opportunities. "People who are connected inside tend to have more job security," he adds.

Even so, mixing business with pleasure requires workers to set personal boundaries.

"If after-hours get-togethers will interfere with your family or personal life, it's imperative to know that and not feel coerced, and to learn how to refuse invitations politely," says Susan Newman, a social psychologist. "Too much socializing leaves you vulnerable to the gossip mill and all manner of office politics you might want to avoid. If you do socialize, be smart about it."

Ileah Foster of Houston did go out with co-workers when she started working as a media trainer. But she quickly discovered that the focus was on drinking and "talking crazy."

"I just didn't want fellowship like that," says Ms. Foster, who also records gospel music with her sister. "I thought there was no point. Now, because they know I generally don't come, I'm no longer invited."

Even so, she finds a silver lining. "Some people think you're a stick–in-the-mud because you don't drink," Foster says. "But others have taken notice of the standard that I've set for my life and respect that."

Beth Shaw, founder of Yogafit Training Systems Worldwide in Torrance, Calif., even goes to church with co-workers. "We discovered a love of all things spiritual," she writes in an e-mail.

For singles especially, after-work gatherings can provide an easy, welcome way to expand their circle of acquaintances. Referring to the Trivia Night outings, Howell says, "We've all become such good friends. A couple people have moved on to other jobs but still come to trivia."

Maria Borda, a fundraiser at the Chemical Heritage Foundation in Phila­delphia, likes to join colleagues for an occasional movie at the end of the day.

"It makes working together a lot more fun when you know each other personally outside of work," she says. "It helps you be invested more with the company when your friends are here. Sometimes it's also the best way to find out what's going on in other departments, what other people are up to...."

Sometimes work-related social activities become a family affair. "We all have children around the same age and we routinely plan weekend outings together," says Dottie DeHart, president of a small public-relations firm in Hickory, N.C. Outings include hiking in the mountains and pool parties.

"It's great for morale," she says. "We have a lot of laughs on Monday about our weekend outing. It also bonds us nicely, as we get to know co-workers' spouses and children. Since we are in the idea business, there are many times when we come up with great ideas while we are all spending time together and relaxing."

For some people, conventions and business trips double as social functions – a way to combine work and play. But whatever the setting, Olson offers a reminder that rules and standards of conduct extend outside the office as well. "When you get outside the traditional workplace, you just have to be careful about how you act, what you say. What at the time seemed like fun on Friday night might be a Human Resource issue on Monday morning." He sums up his advice with a maxim: All things in moderation.

For those who are hesitant to socialize with colleagues, Mr. Singer offers this encouragement: "The benefits of having real relationships, real friendships with other people will outweigh any pitfalls."